1. Field of the Invention
The present invention relates to a composite sheet and a pneumatic tire, and more particularly to a pneumatic tire having high puncture resistance and excellent stability and maneuverability even under zero inflation pressure.
2. Description of the Related Art
The composite sheet of present invention is an improvement to ply sheet. Ply sheet has high tensile modulus along its cord direction but low bending stiffness and, it can only bear small compression load. The flexibility of ply sheet makes it very easy to be formed to a double curvature surface of desirable shape. It is widely used in structure that defines partial or whole surface of a confined air chamber. Tire and hose are good examples of this type of industry applications. This type of structure can tolerate large deformation and absorb vibration. However, the structure depends on internal inflation pressure to form a stable condition to bear the large working load. When inflation pressure is lost for some reason, the structure becomes unstable and the working load has to be passed to adjacent structure. Among other drawbacks of this type of structure are puncture/cut prone and low compression load bearing ability.
In recent years, many so-called run-flat tires have been patented. They can be categorized as three types: the first type is to add crescent shape reinforcing rubber layer and/or reinforcing cord layer at sidewall portions and/or tread portion (U.S. Pat. No. 6,209,604); the second type is to add safety insert to the rim assembly(U.S. Pat. No. 6,039,099); the third type is to embed metallic rings at tire shoulder portions or at tire crown potion (U.S. Pat. No. 6,276,416).
In the first type of run-flat tire, the thickness of rubber at sidewall portion has been greatly increased in order to provide sufficient support to tire under flat tire condition. As a result, the ride comfort and heat buildup inside tire and deterioration in dynamic tire performance under normal condition become trade-offs in this type of tire.
In the second type of run-flat tire (tire, safety insert and rim assembly), tire will lay flat on the safety insert with loss of inflation pressure. In this flat tire situation, tire sidewall portion can carry little load and most of load is transferred through the contact between tire tread and road and the contact between tire inner surface and safety inserts. As a result, the tire handling ability becomes hard to predict under flat tire condition, especially when vehicle is cornering.
The third type of run-flat tire is almost impractical. It is hard to find any metallic material to make a ring which can tolerate such cyclic large deformation which tire has experienced at normal working condition. Further more, this type of tire is unstable under flat tire condition since flexible tire structure is not capable of transferring the load between rings and rim under this condition.
In some previous US patents, people claimed achieving certain improvement of puncture resistance of ply by selecting different construction of cord and even by replacing cord with cable (a structure comprises of several twist cords). In their claimed methods, bigger diameter size of filaments was selected and, as a result, the larger bending stiffness of the cord is obtained. However, the increment of bending stiffness of cord is undesirable in some engineering applications. Further more, the improvement of puncture resistance of the ply structure in those patented inventions is not sufficient to prevent the failure of the structure in most cases.